Proton exchange membrane fuel cells (PEMFCs) are considered to be clean and efficient power sources. Steele et al., Nature 2001, 414, 345. However, the high cost and unsatisfactory durability of catalysts are major barriers for large-scale commercialization of PEMFCs. Borup et al., N. Chem Rev 2007, 107, 3904. By switching the polymer electrolyte from an “acidic” condition to a “basic” one, HEMFCs are able to work with non-precious metal catalysts and the catalysts are expected to be more durable. Other cheaper fuel cell components are also possible such as metal bipolar plates. Varcoe, et al., Fuel Cells 2005, 5, 187; Gu et al., Angew Chem Int Edit 2009, 48, 6499; Gu et al., Chem Commun 2013, 49, 131. However, currently available HEMs and HEIs exhibit low alkaline/chemical stability, low hydroxide conductivity, high water uptake, and low mechanical integrity under dry conditions, especially after wet-dry cycles.
The biggest challenge for HEMs/HEIs at present is achieving a high chemical stability at desired operation temperatures of 80° C. or more, and ideally 95° C. or more (e.g., in the presence of nucleophilic hydroxide ions). Varcoe et al., Energ Environ Sci 2014, 7, 3135. The most commonly encountered cationic functional groups (e.g., benzyl trimethyl ammonium and alkyl chain ammonium) can undergo a number of degradation processes in the presence of hydroxide ions nucleophiles by direct nucleophilic substitution and Hofmann elimination. Moreover, the polymer backbone of most base polymers for HEM/HEI applications (e.g., polysulfone and poly(phenylene oxide)) unavoidably contains ether linkages along the backbone, which makes the HEMs/HEIs potentially labile under high pH conditions. Lee et al., Acs Macro Lett 2015, 4, 453; Lee et al., Acs Macro Lett 2015, 4, 814. The strongly nucleophilic hydroxide ions attack these weak bonds and degrade the polymer backbone. Thus, alternative cationic groups, organic tethers, and polymer backbones are needed to enhance chemical stability of HEMs/HEIs.
Another concern regarding current HEMs/HEIs is their hydroxide conductivity. In comparison to Nafion, HEMs have intrinsically lower ionic conductivities under similar conditions, because the mobility of OH− is lower than that of H. Hibbs et al., Chem Mater 2008, 20, 2566. Greater ion-exchange capacity (IEC) is needed for HEMs/HEIs to achieve greater hydroxide conductivity. However, high IEC usually leads to a membrane having high water uptake (i.e., a high swelling ratio), decreasing the morphological stability and mechanical strength of the membrane, especially after repeated wet-dry cycles. This highly swollen state when wet is a major reason for decreased flexibility and brittleness of HEMs when dry. The removal of the trade-off between high hydroxide conductivity and low water uptake has been a major setback in designing high-performance HEMs/HEIs. Pan et al., Energ Environ Sci 2013, 6, 2912. Chemical cross-linking, physical reinforcement, side-chain polymerization, and block-copolymer architecture have been tried to reduce water uptake while maintaining acceptable hydroxide conductivity, but these techniques bring challenging problems, e.g., reduced mechanical flexibility, decreased alkaline stability, and/or increased cost. Gu et al., Chem Commun 2011, 47, 2856; Park et al., Electrochem Solid St 2012, 15, B27; Wang et al., Chemsuschem 2015, 8, 4229; Ran et al., Sci Rep-Uk 2014, 4; Tanaka et al., J Am Chem Soc 2011, 133, 10646. Additionally, almost all side-chain or block-copolymer HEMs are based on flexible aliphatic polymer chains due to limited available synthesis methods. As a result, the membranes still cannot provide morphological stability (low swell ratio) at high IECs and high temperature. Wang et al., Chemsuschem 2015, 8, 4229; Ran et al., Sci Rep-Uk 2014, 4; Marino et al., Chemsuschem 2015, 8, 513; Li et al, M. Macromolecules 2015, 48, 6523.
An additional obstacle to using HEMs is achievement of mechanical flexibility and strength in an ambient dry state. Most HEMs exhibit low mechanical strength and are very brittle in a completely dry state especially after being completely swollen. It is difficult to obtain and handle thin membranes that are large in size as needed for commercial use of HEMs. Without good mechanical properties, the ionomers cannot form and keep an adequate triple phase structure in the fuel cell electrode at high temperature, such as at or above 80° C. Li et al., J Am Chem Soc 2013, 135, 10124.
Another highly desirable feature of an HEI is that the polymer be soluble in a mixture of lower boiling alcohol and water but insoluble in pure alcohol or water so that the HEIs can be readily incorporated into an electrode catalyst layer yet not be dissolved away by water or alcohol.